This invention relates to a window structure for transmission of radiation, such as X-rays, to radiation detector elements.
X-ray detectors are used in a variety of situations including electron microscopy, X-ray telescopy, and X-ray spectroscopy. Each situation may subject the detector to different environmental and operating conditions such as atmospheric pressure on the equipment, various energy levels of the radiation, etc. For example, some energy-dispersive detectors must be operated in a vacuum. If the detector is used in an electron microscope, then it will be subjected to and must be able to withstand scattered high-energy electrons. For proton induced X-ray emission detection, the detector must withstand scattered high-energy protons.
X-ray detectors typically include in the structure some type of window or receptor for receiving and passing radiation to detector elements. The window structure includes a piece of material for passing the desired radiation and filtering or blocking undesired radiation, where the material is placed over an opening or entranceway to the detector. Exemplary materials which have been used in the past include beryllium, alumized polypropylene, silicon nitride, silicon, boron nitride, boron, and polyethylene terphthalate (mylar), all formed into a film or sheet to cover and span the required opening. Because of the size of the openings to be covered in prior art structures, typically six mm wide, the films must be formed thick enough to withstand pressures to which the detector would be subjected, gravity, and normal wear and tear from use of the detector. However, the thicker is the film, the more absorptive it is so that some radiation which the user desires to detect might be absorbed by a film which is too thick. For example, the longer are the X-ray wave lengths, the more likely they are to be absorbed by a thick film. It is therefore desirable to provide a window film which is as thin as possible but yet sufficiently thick and sturdy to span the opening to be covered, and to withstand differential pressure--e.g., at least one atmosphere.
One approach to meeting the need of providing a thin film which is capable of spanning radiation entrance openings is to utilize a screen or mesh as a film support. In other words the screen or mesh is placed over the opening and then the film is placed on the screen or mesh to be supported thereby. This type of support structure, however, has a number of drawbacks, the primary one being that the screens and meshes are rough and coarse and thus, at the locations they contact the film, the film is caused to stretch, weaken and burst. Increasing the thickness of the film to compensate simply results in increasing the absorptive characteristics of the film so that certain radiation cannot be detected. Another disadvantage of the use of screens and meshes is that they themselves can break under pressure. Making screens and meshes stronger by thickening the wires (and making smaller openings) results in the undesired blockage of more radiation.